This invention relates to a polyurethane material for decorative parts. More particularly, this invention relates to a material suitable for the formation of a transparent synthetic resin part possessing a positive meniscus on a base of synthetic resin in interior and exterior decorative parts of automobiles requiring exacting lightfastness such as, for example, steering wheels, ornaments and marks for incorporation in interior wall materials, and various moldings including side moldings, bumper moldings locker moldings, and interior ornamental strips.
As a typical representative of the decorative marks enumerated above, a mark 1 illustrated in FIG. 1 which is intended for incorporation in a steering wheel will be described below. It should be noted that the mark is selected purely for illustration and is never meant as a limitation on the invention.
This mark 1 is produced by a method which comprises casting a liquid transparent synthetic resin material of suitable viscosity on an upper surface 3a of a disc-shaped base 3 made of synthetic resin or metal and provided with an edged periphery and then allowing the cast resin to set thereby giving rise to a transparent synthetic resin part 5 possessing a positive meniscus (so-called potting method) (Japanese Patent Application Disclosures SHO No. 58(1983)-221,741 and SHO No. 61(1986)-207,249). As the material for the transparent synthetic resin part 5 mentioned above, it has been customary to employ a two-liquid type polyurethane material using a polyol component and an isocyanate component with a view to ensuring resistance to abrasion and resistance to shock, for example. The aforementioned polyol component generally incorporates a combination of various stabilizers therein for the main purpose of preventing the produced mark from yielding to discoloration or development of cracks under the influence of solar rays and ozone in the air.
As stabilizers mentioned above, hindered amine type ultraviolet light stabilizers represented by bis(2,2,6,6-tetramethyl-4-piperidyl)sebacate and hindered phenol type antioxidants represented by pentaerythrityl-tetrakis-[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate] have been generally used in combination.
The inventors' test performed on the aforementioned combination of stabilizers, however, has revealed that long retention of good appearance with this combination of stabilizers is difficult to achieve (Comparative Experiment, Table 2). To be specific, products using such stabilizers show signs of discoloration and surface fusion (ready impartation of finger prints on the surface of a test piece) after the test of exposure to weather conditions or the test of exposure to light.